1. Field of the Invention
The present invention relates to an optical scanning apparatus and, more particularly, to an optical scanning apparatus ideally suited to, for example, a laser beam printer (LBP) apparatus and a digital copying machine having an electrophotographic process wherein a luminous flux optically modulated and emitted from a light source means is deflectively reflected by an optical deflector composed of a rotary polygon mirror or the like, then applied onto a plane to be scanned through an image optical system having f.theta. characteristics (an f.theta. lens unit) in order to optically scan the plane to record image information.
2. Description of Related Art
In a conventional optical scanning apparatus used for a laser beam printer or the like, a luminous flux optically modulated and emitted from a light source means according to an image signal is periodically deflected by an optical deflector constituted by, for example, a rotary polygon mirror, then it is converged on a surface of a photosensitive recording medium (photosensitive drum) into a spot shape by an image optical system having f.theta. characteristics, so that the surface is optically scanned to record images.
FIG. 1 is a schematic cross section in the main scanning direction of an essential section of a conventional optical scanning apparatus. In the drawing, a divergent luminous flux emitted from a light source means 11 is formed into a nearly parallel beam through a collimator lens 12, and it is restricted in terms of the quantity of light by a stop 13 before being launched into a cylindrical lens 14 having a predetermined refractive power only in the sub-scanning direction. The parallel luminous flux launched into the cylindrical lens 14 comes out in the main scanning cross section as is, whereas it is converged in the sub-scanning cross section to form a nearly line-shaped image on a deflection surface, i.e. a reflection surface, 15a of an optical deflector 15 composed of a rotary polygon mirror.
The luminous flux deflectively reflected by the deflection surface 15a of the optical deflector 15 is guided to the plane of a photosensitive drum 18 serving as a plane to be scanned through an image optical system (an f.theta. lens unit) 16 having f.theta. characteristics. As the optical deflector 15 is rotated in the direction indicated by arrow A, the surface of the photosensitive drum 18 is optically scanned to record the image information.
For this type of optical scanning apparatus to achieve highly precise recording of image information, it is required that the field curvature be successfully compensated for, the spot diameter be uniform over the entire plane to be scanned, and the distortion aberration exhibit f.theta. characteristics wherein the angle of incident light and image height have a proportional relationship. Various optical scanning apparatuses having such optical characteristics or compensation optical systems (f.theta. lens units) therefor have been proposed in the past.
With an increasing trend toward more compact design and lower cost of laser beam printers, digital copying machines, etc., optical scanning apparatuses are also being required to be made more compact and less expensive.
Various optical scanning apparatuses having an f.theta. lens unit composed of a single lens which satisfy the demands mentioned above have been proposed; some of them have been disclosed in, for example, Japanses Patent Publication No. 61-48684, Japanses Patent Laid-Open No. 63-157122, Japanses Patent Laid-Open No. 4-104213, and Japanses Patent Laid-Open No. 4-50908 which corresponds to U.S. Pat. No. 5,111,219.
Among these proposed apparatuses, the ones disclosed in Japanses Patent Publication No. 61-48684 and Japanses Patent Laid-Open No. 63-157122 employ, as the f.theta. lens unit, a single lens having a concave surface facing an optical deflector to focus the parallel beam from a collimator lens onto the plane of a recording medium. The apparatus disclosed in Japanses Patent Laid-Open No. 4-104213 employs a single lens, which has a concave surface facing the optical deflector and a toroidal surface facing an image surface, as the f.theta. lens into which a luminous flux converted to a convergent light beam through the collimator lens is launched. The apparatus proposed in Japanses Patent Laid-Open No. 4-50908 corresponding to U.S. Pat. No. 5,111,219 uses a single lens having high-order aspherical surfaces as the f.theta. lens into which a luminous flux converted to a convergent light beam through the collimator lens is launched.
The conventional optical scanning apparatus disclosed in Japanses Patent Publication No. 61-48684, however, poses a problem in that it cannot completely eliminate the field curvature in the sub-scanning direction, and that the focal length f from the f.theta. lens to a plane to be scanned is long because a parallel beam forms an image on the plane to be scanned, thus making it difficult to implement a compact optical scanning apparatus.
The apparatus disclosed in Japanses Patent Laid-Open No. 63-157122 is disadvantageous in that the f.theta. lens is thick, making it difficult to fabricate it by molding with resultant higher cost.
The apparatus proposed in Japanses Patent Laid-Open No. 4-104213 has a problem in that there is distortion aberration remaining, and that jitters are produced at intervals corresponding to the number of the polygon facets due to a mounting error of a polygon mirror serving as the optical deflector.
The apparatus disclosed in Japanses Patent Laid-Open No. 4-50908 has a high-order aspherical f.theta. lens to successfully compensate for aberrations; however, the spot diameter in the sub-scanning direction tends to change according to the height of an image because of uneven magnification in the sub-scanning direction between the optical deflector and the plane to be scanned.
In addition to those described above, optical scanning apparatuses using two lenses to constitute the f.theta. lens unit have been proposed in, for example, Japanses Patent Laid-Open No. 56-36622 and Japanses Patent Laid-Open No. 61-175607. The sections of the f.theta. lenses of these apparatuses are a spherical or slightly aspherical, making it difficult to attain more compact design, lower cost, and higher precision.